1. Field of the Invention
The present invention relates to a valve-integrated transponder that is integrated with an air valve fixed to a valve hole of a wheel rim and is disposed in a tire, and that is used in a system for monitoring the air pressure of the tire.
2. Description of the Related Art
In recent years, more and more vehicles are equipped with a system that enables a driver at the driver seat to monitor the air pressure of the tires. Specifically, such a system is provided for allowing a driver to quickly check the air pressure of the tires and to see whether the air pressure is abnormal while driving a vehicle, and is realized by installing transponders equipped with detecting elements and antenna elements in appropriate positions within the tires. A transponder of this type is integrally joined to, for example, an end of an air valve that is located within a tire. See Japanese Unexamined Patent Application Publication No. 2006-69389 (pages 3 to 5, and FIG. 1) for an example of such a valve-integrated transponder. In this case, the air valve may be of a clamp-in type that is screwed onto a wheel rim or a snap-in type that is press-fitted to a valve hole of a wheel rim. As described in Japanese Unexamined Patent Application Publication No. 2006-69389, an air valve of a snap-in type is advantageous in that it can be readily fitted to a wheel rim. Although the type of built-in antenna to be used in a transponder contained inside a tire is not particularly limited, an inverse F antenna element made by forming a metal plate is advantageous in terms of low fabrication cost and compactness.
A valve-integrated transponder of the related art normally contains a battery power source. However, with a structure equipped with a battery, the frequency of detecting the air pressure of a tire will need to be minimized to expand the life of the battery, and a complicated process will inevitably be necessary when replacing the battery. Recently, a valve-integrated transponder that does not require a battery power source has been proposed. Specifically, in this valve-integrated transponder, the antenna element inside the tire is excited by a radio wave sent from an external antenna in the vehicle body and is supplied with a signal current based on information detected by the detecting element. Such a valve-integrated transponder not requiring a battery can allow for an increase in the frequency of detecting the air pressure of the tire and can thus achieve higher detection accuracy. In addition, the valve-integrated transponder also allows for lower maintenance costs due to not requiring a battery replacement process.
Generally, a radio wave radiating from an antenna element of a valve-integrated transponder is transmitted to an external antenna in the vehicle body by passing through a side wall of a tire. If the radio wave used has a short wavelength and has a strong property of rectilinear propagation, such as a 2.4 GHz radio wave used in a transponder that does not require a battery, it is necessary to increase radiation components of a direct wave directed from the antenna element in the tire towards the side wall. However, with a common inverse F antenna element, a radio wave radiates both upward and sideward from a radiation conductor. This makes it difficult for the radio wave to radiate efficiently towards the side wall of the tire. Regarding a valve-integrated transponder fitted to a snap-in type air valve, the transponder receives a large amount of force when the air valve is being press-fitted to the valve hole of the wheel rim. To enhance the mechanical strength of the transponder, it is desirable to provide a casing, which houses an antenna element and the like, with a reinforcement rib that extends along an extension of the axis of the air valve. However, providing such a reinforcement rib in a manner that it does not come in physical contact with the common inverse F antenna element is problematic in that the casing becomes large in size.